A hard disk drive (HDD) is an information storage device which has grown in popularity and has become almost indispensable in computers and various household electronics products, particularly for applications involving mass information storage. In a HDD, a magnetic recording method is employed, in which information is recorded in a magnetized state of a ferromagnetic thin film (magnetic recording film) formed on a recording medium. The magnetic recording film is an aggregate of minute magnetic particles, in which each magnetic particle is given a magnetic characteristic capable of stably maintaining one of two magnetized states corresponding to the recording information. Also, the adjacent magnetic particles are formed sufficiently separated from each other such that each magnetic particle can indicate an independent magnetized state irrespective of the surrounding magnetic particles.
In a conventional magnetic recording method, magnetic particles making up the magnetic recording film have various sizes, and their arrangement is random. In forming the magnetic recording film through a thin film formation process, such as coating or sputtering, the growth or arrangement of magnetic particles is determined through a local formation process for the magnetic recording film, whereby such non-homogeneity is generally inevitable. The structure of these types of magnetic recording films is described in Japanese Patent Office (JPO) Pub. No. JP-A-2005-190552, as just one example. Also, one recording information unit (called one bit) is recorded for a group of many adjacent magnetic particles, using a magnetic head. At this time, the serration along the shape of each magnetic particle in a magnetization transition area of the recording bit appears as a “transition noise.” However, variations in the size or arrangement of each of the magnetic particles are averaged by allocating many magnetic particles to one bit, whereby a recordable and reproducible system can be constructed without error. That is, if the magnetic particle is sufficiently small for the size of the recording bit, a deterioration in the signal quality does not become noticeable.
With the higher recording density used in HDD in recent years, there is a demand for making the area of forming one hit smaller. To maintain the number of magnetic particles per bit, the area of a magnetic particle is also reduced. However, in recent years, it has become more difficult to make the magnetic particles smaller, as a limit is fast approaching on the minute size already being used, e.g., the magnetic particles soon will not be able to be made smaller without losing their ability to hold information.
This effect occurs because the magnetized state of the magnetic particle is always subject to a disturbance owing to ambient thermal energy. When the environmental temperature is T, the magnitude of thermal disturbance energy is estimated to be kBT (kB is the Boltzmann constant). On the other hand, the stability of the magnetized state of the magnetic particle is estimated in terms of KuV. Herein, Ku is a magnetic anisotropy energy of the material composing the magnetic particles, and V is the volume of magnetic particle. If the ratio KuV/kBT is smaller, the stability of the recording information is impaired. Ku has a restriction due to material selection or the magnetic field required for recording. Also, kBT is determined depending on the temperature (around 300K) at which the HDD is used. Accordingly, the fine quality of the magnetic particles (smaller V) necessarily leads to a decrease in KuV/kBT, making it impossible to keep the recording information stable, whereby the function of the recording medium is lost.
The above problem cannot be avoided using conventional magnetic recording methods, and at present is an important factor in deciding the upper limit of the recording density of a HDD. To overcome this problem, a “bit pattern medium,” presupposing that one magnetic particle is allocated to one bit, is regarded as promising in the recording medium technology of the next generation. The particular features of a bit patterned medium are described in JPO Pub. No. JP-A-2008-123638 and IEEE Trans. Magn., vol. 43, p. 2142 (2007), but a quick summary of this technology follows.
A magnetic particle in bit pattern medium, which is essentially different from a conventional magnetic particle, is an aligned pattern dot. To represent the recording bit with one magnetic particle or a smaller number of magnetic particles, the size and arrangement of magnetic particles cannot be random, as they conventionally are arranged. The size and arrangement are controlled with precision by the recording and reproducing system. With the precise size and arrangement, the recording position of information can be correctly determined or the recorded information can be read as a positional signal.
On the other hand, the pattern dot is the minimum unit of magnetization reversal. If a plurality of regions having different magnetized states exist within the pattern dot, the correspondence between the recording information and the pattern dot is not detectable, so that the correct recording and reproducing cannot be made.
By satisfying the above prerequisites, the area of one magnetic particle (pattern bit) can be put closer to the area occupied by one bit in the bit pattern medium than in conventional recording schemes. As a result, the volume of pattern dots in the magnetization reversal unit is significantly larger than the conventional volume; thereby, it is possible, in principle, to realize a higher recording density while maintaining thermal stability of the head and HDD.
In IEEE Trans. Magn., vol. 43, p. 2142 (2007), a detailed examination for the recording and reproducing process for a bit pattern medium by simulation is made. In performing magnetic recording on the pattern dots formed on the substrate, switching of the direction of recording magnetic fields in synchronism with the pattern dot position is employed. It is noted that the recording synchronization timing lag remarkably increases the recording error on bit pattern medium. If the pattern dot position on the recording medium is displaced from the intended position because of a problem associated with the pattern formation process, there is the same influence. Also, recording quality is remarkably degraded due to head misregistration (tracking misregistration) in the track transverse direction.
In IEEE Trans. Magn., vol. 43, p. 2142 (2007), the reason why there is a very strict restriction on the recording synchronization timing or pattern misregistration is that a magnetization reversal field dispersion of the pattern dot increases due to a magneto-static interaction with the adjacent pattern dot. The relationship between the magnetization direction of the pattern dot having already completed recording and the pattern dot on an adjacent track and the magnetization direction of the pattern dot subject to recording from now is always different. Accordingly, the magneto-static interaction between these pattern dots brings about an unpredictable variation in the effective recording magnetic field, causing a recording error. In JP-A-2008-123638, a method for solving this problem is disclosed in which an exchange interaction acting in the opposite direction to the magneto-static interaction is introduced between the pattern dots.
In addition, a method for constructing the single pattern dot in which two portions having different magnetic characteristics are closely contacted and magnetically bonded with exchange interaction is described below. For bit pattern medium, the pattern dot is the minimum unit of magnetization reversal, as previously described. Accordingly, the two portions are bonded by the exchange interaction stronger than the magneto-static interaction.
In J. Appl. Phys., vol. 100, p. 074305 and J. Appl. Phys., vol. 103, p. 07C504, a structure in which two magnetic layers having different anisotropy magnetic fields (Hk) are laminated in a direction perpendicular to a substrate plane is disclosed. Within the pattern dot having this structure, the portion having smaller anisotropy magnetic field starts the magnetization reversal ahead and then the magnetization reversal of the portion having larger anisotropy magnetic field is induced. In the sense of the structure for realizing such a non-uniform magnetization reversal process, this structure is called an exchange spring structure. With this exchange spring structure, it is possible to realize the magnetization reversal of the pattern dot in the small magnetic field, while maintaining the relatively high thermal stability (total sum of KuV in the upper and lower layers), as theoretically disclosed in J. Magn. Magn. Mater. Vol. 290-291, p. 551 (2005). Thereby, it is expected that high recording density can be realized by making the pattern dots finer, while suppressing the head magnetic field required for recording.
In The Abstracts Book of The 52th Annual Conference on Magnetism and Magnetic Materials, GC-08, there is a description of a structure in which the single pattern dot is formed by two portions having different magnetization reversal fields (Hk). However, the document discloses that the structure in which two portions having different magnetization reversal fields Hk are closely contacted in the parallel direction to the substrate plane, not in the perpendicular direction. In this case, a non-uniform magnetization reversal mode peculiar to the exchange spring structure acts, whereby it is possible to reduce the head magnetic field required for recording, while maintaining the relatively high thermal stability.
Therefore, to achieve higher recording density of a few Tb/in2 class in the bit pattern medium, as described above, is difficult while maintaining other desired properties of the head and HDD, such as decreasing the magneto-static interaction with adjacent pattern dots as much as possible, and suppressing a variation in the effective magnetization reversal field of the pattern dots caused by a difference in the surrounding magnetized state. The method of introducing the exchange interaction acting in an opposite direction to the magneto-static interaction between the pattern dots as proposed in JPO Pub. No. JP-A-2008-123638 can solve this problem, in an ideal state. However, if the exchange interaction between the pattern dots is not uniform, it is difficult to attain the desired effect, or rather, there is a danger that the dispersion of the magnetization reversal field increases. To obtain the uniform exchange interaction, it is required that the magnetic material for connecting the pattern dots is formed quite precisely, and the level of precision is not easy to obtain.
Generally, if each pattern dot shape on the substrate plane is identical, it is possible to suppress the magneto-static interaction between the pattern dots relatively easily by reducing the thickness of the magnetic recording film composed of the pattern dots. For example, in a bit pattern medium in which the pattern dots (square shape, 20 nm×20 nm) are arranged every 25 nm on the substrate plane, if the thickness of the magnetic recording film is reduced from 20 nm to 5 nm, the magneto-static field is decreased to one-half or less. Also, for fixed regular intervals of pattern dots, the magneto-static interaction can also be decreased by reducing the area of the pattern dot to increase the gap between pattern dots. In the previous example, if the size of pattern dot is reduced from the square of 20 nm×20 nm to a square of 10 nm×10 nm, with the regular intervals of pattern dots fixed, the magneto-static field is also decreased to one-half or less.
However, in the previous examples, the volume V of each pattern dot is greatly decreased. If the volume V is smaller, it is required that the magnetic anisotropy energy Ku is increased to maintain KuV, that is a thermal stability index, and it is inevitable that a material having a greater magnetization reversal field will be used, presupposing that the head magnetic field required for recording is increased. On the other hand, in consideration of reproduction of the magnetic information recorded on the pattern dot, the reproducing signal strength is greatly decreased, bringing about a danger that the signal to noise ratio (SNR) is deteriorated so much that reading the information may be impossible, especially when the spacing between the pattern dots is larger.
In light of these circumstances, a magnetic medium and recording device, such as a HDD, which can have an increased margin for the recording synchronization timing lag or tracking misregistration on the pattern dot by combining such a structure so that the recording error is less likely to occur, even if the magneto-static interaction itself exists, would be very beneficial to the recording industry.